Down syndrome (DS) is a condition of intellectual disability characterized by accelerated aging. The broad, long-term objective of this project is to identify the cause(s) of aging in Down syndrome, with a particular emphasis on investigating the role of DNA repair in the aging phenotype observed in Down syndrome. The hypothesis is that the aging phenotype of Down syndrome results from lifelong inhibition of BER induced by chromosome 21-linked miRNA overexpression. This hypothesis will be tested in the following Specific Aims: Specific aim 1: Determine whether miR-155 and/or miR-802 stable overexpression alone or in combination recapitulate the BER and senescence phenotypes of Down syndrome (DS). Pol promoter activity, BER capacity, and senescence will be evaluated. The ability of MeCP2 and/or Creb1 expression to restore BER will also be evaluated. In parallel the impact of pol nullizygosity on senescence induction will be determined. This will allow a directly connection between BER loss and senescence to be established. Specific aim 2: Determine whether silencing of either miR-155 or miR-802 in primary DS fibroblasts reverses the DS-induced inhibition of BER and whether that then prevents early senescence. These data would directly tie chromosome 21-mediated miRNA overexpression to BER capacity and senescence. Further, to test whether miRNA overexpression inhibits BER through MeCP2-mediated signaling, MeCP2 and CREB1 will be overexpressed in DS lines to determine both whether this ameliorates the BER phenotype of DS and whether either may be an appropriate interventional target. Specific aim 3: Down syndrome provides a unique opportunity to investigate the role of senescence as a barrier to tumorigenesis in a relevant, in vivo model. The Ts65DN mouse model of DS is known to recapitulate much of the DS phenotype, including accelerated aging (to the extent it has been evaluated) and premature senescence in ex vivo fibroblasts. Thorough pathological and morphological analysis of tissues over time will be evaluated in this model. In addition we will determine miR-155 and miR-802 expression, pol/BER capacity and senescence in a panel of tissues to begin to identify which tissues exhibit aging phenotypes in the DS model. These experiments will generate critical information to justify further aging studies and interventional strategies in this mouse model.